This research program will examine whether moderate aerobic exercise provides specific benefits for building stress-resistance in older adults (ages 55 - 74). The specific aims are to determine (1) if habitual physical activity moderates the emotional and physical health consequences of stressful life events, (2) whether habitual physical activity and age are associated with the psychophysiological changes elicited by laboratory stressors, (3) if a single session of aerobic exercise alters acute mood and psychophysiological responsiveness to laboratory stressors, and (4) whether an aerobic exercise training program influences emotional well-being and psychophysiological reactivity. Three studies are proposed. Study 1 is a prospective study of 192 subjects. Fitness scores from a maximal bicycle ergometer fitness test and self-reports of stressful life experience will be used to predict emotional maladjustment and physical health difficulty over a subsequent 6-month period. Interactions between fitness and stress, which will confirm that fitness moderates the deleterious effects of stress, are predicted. Study 2 is an investigation of psychophysiological functioning. Challenging stressors will be administered in a laboratory setting to 144 subjects, and cardiovascular measures of autonomic functions and electroencephalogram (EEG) measures cortical activation will be examined. The subjects will participate in either a stationary cycling or a control condition for 20 minutes before changes in autonomic reactivity and EEG measures are examined. Subjects with high fitness levels (assessed in Study 1) are expected to show less autonomic reactivity than low-fit subjects. The single session of aerobic exercise is expected to alter mood and produce beneficial changes in EEG parameters. Study 3 is an experimental comparison of an aerobic exercise program and a flexibility training condition. Forty subjects will participate in each condition for 16 weeks. Dependent measures of emotional well-being, physical health, psychophysiological reactivity, and maximal oxygen consumption during a graded exercise test will be examined. The aerobic training condition is expected to lead to greater improvements in emotional well-being and attenuated autonomic reactivity than the flexibility training condition. Collectively, these studies directly test the utility of using aerobic exercise for enhancing stress-resistance in older adults.